The present invention relates in general to fluidized bed boilers and, in particular, to a new and useful apparatus for feeding solids to the fluidized bed of a fluidized bed boiler.
The combustion zone of a circulating fluidized bed (CFB) boiler 10, as shown in FIG. 1, is divided into two parts; the primary zone 12 and the furnace or secondary zone 18. The primary zone, located below the furnace, is the area where the circulating fluidized bed solids are re-injected back into the combustion zone by a particle separator 14 and return line 16. A non-mechanical seal or valve 32 is provided in return line 16. The primary zone is also where fuel and sorbent for sulfur retention (if required) is introduced. The primary zone acts as a distribution zone for solids (CFB solids, fuel and sorbent) so they are evenly distributed across the primary zone and furnace and as a preliminary combustion zone. Fifty to 100% of the total combustion air is fed into the bottom of the primary zone at a windbox 20. The remaining combustion air is fed in through wall ports 22. These wall ports define the separation between the primary zone 12 and the furnace 18. Combustion is completed in the furnace. The primary zone has a refractory lining 24 as it is exposed to a reducing atmosphere. The furnace is refractory lined only in high erosion areas. The boiler also includes a convection pass 30 for the hot exhaust gases.
A major problem area for CFB boilers is in firing high volatile or highly reactive fuels such as wood. The usual means of feeding fuel into the primary zone of a CFB boiler is with a screw conveyor 26 which pushes the fuel in through a wall port called the fuel feed point 28. A highly reactive fuel will devolitize in the area immediately around the fuel feed point. This results in a plume of combustible gases immediately over the fuel feed point. These concentrated combustible gases cannot readily mix with the combustion air because the air is evenly distributed across the cross section of the primary zone and the furnace. The result is a temperature gradient across the unit due to the combustion being concentrated at the combustible gases plume above the fuel feed point. This high temperature zone encourages NOx formation. Also, the poor mixing of the combustible gas with the combustion air can lead to low combustion efficiency, high CO emissions and combustion occurring in the solids separator and in the convection pass.
What is needed is a close, intimate mixing of fuel, air and circulating bed solids in a way such that they are evenly distributed in the primary zone and that combustible gases, air and circulating bed solids are intimately mixed and evenly distributed in the furnace.
U.S. Pat. No. 4,552,203 to Chrysostome, et al discloses a fluidized bed reactor having a particle return and supply mechanism which includes a feed screw and conduit that receives both cold and hot portions of the particles being returned to the fluidized bed. Gas is injected along the length of the return conduit for suspending and conveying the solid particles.
A fluidized bed having an inlet zone which is positioned laterally of the combustion zone, is disclosed in U.S. Pat. No. 4,585,706 to Klaschka.
A boiler with a fluidized bed which is divided into a deep part with walls inclined toward a lower outlet, and a shallow part above the deep part, is disclosed in U.S. Pat. No. 4,528,945 to Virr, et al. Fuel is supplied by a feed screw near the top of the upper shallow part of the bed.
Other U.S. patents showing fluidized beds where the fuel is supplied at a relatively high location in the bed area include U.S. Pat. Nos. 4,446,629 to Stewart, et al; 4,539,939 to Johnson; and 4,542,716 to Dreuilhe, et al.
U.S. Pat. No. 4,594,967 to Wolowodiuk discloses a fluidized bed which is divided into separate bed portions.